Syrian detainees who took part in anti-government protests sit in a courtroom before their release in September 2012 in Damascus, Syria. Intelligence agents are grabbing civilians seen as a threat to President Bashar Assad’s regime, including human-rights activists and lawyers, and are delivering them to torture dungeons where they vanish.

BEIRUT — About 30 security agents showed up just after midnight, breaking down the door to an apartment in the town of Daraya near the Syrian capital of Damascus. They grabbed a 24-year-old university student and drove off.

That was a year ago. The young man, who had been providing aid to Syrians displaced by the country’s civil war, was never heard from again. His family was told by former prisoners that he ended up in one of the torture dungeons of President Bashar Assad’s regime. They don’t know if he’s dead or alive.

More than two years into the conflict, such accounts have become chillingly familiar to Syrians. Intelligence agents have been seizing people from homes, offices and checkpoints, and human rights activists say the targets often are peaceful regime opponents, including defense lawyers, doctors and aid workers.

Syrian human rights monitors say the number of those disappeared without a trace is now in the thousands. By comparison, the official figure of those who disappeared in Argentina’s “dirty war” of the 1970s and 1980s is about 13,000, though rights activists say the actual figure is more than twice that.

In such “enforced disappearances,” governments refuse to acknowledge detentions or provide information about those taken. The point traditionally is to get rid of opponents and scare the rest of the population into submission — a rationale laid out in Adolf Hitler’s “Nacht und Nebel (Night and Fog)” decree of 1941.

In Syria, the goal is to “terrorize the society and dry up the revolution,” said Anwar al-Bounni, a veteran defense lawyer and human rights campaigner in Damascus. “The regime focuses on arresting peaceful activists to turn it purely into an armed conflict.”

Four Syrian human rights monitors offered separate estimates ranging from about 10,000 to as many as 120,000 disappeared. The two lower estimates are based on information from families and released prisoners, while the higher figures are based on extrapolation from partial data.

Two international groups, Amnesty International and Human Rights Watch, said they think a majority of detainees in Syria are held under conditions amounting to enforced disappearance. Amnesty said it estimates that tens of thousands of Syrians are in detention but does not have exact figures.

The wide range of numbers also reflects the difficulty of collecting information at a time of chaos, on a practice the regime doesn’t acknowledge.

A U.N. panel said in a 2013 report that when it asked about allegations of thousands of enforced disappearances in Syria, the Assad government responded that “there were no such cases in Syria” and that all arrests were being carried out legally.

In Damascus, al-Bounni, the defense lawyer, said he personally knew of hundreds who had disappeared, some for weeks or months and others whose fate remains unknown.

The Violations Documentation Center in Syria, one of the rights monitors, said nearly 2,400 detainees have been killed in custody since March 2011, including 1,375 by torture.

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